This invention relates to accelerometers in general and more particularly to an improved temperature compensation mechanism for a accelerometer utilizing proof masses disposed in a damping fluid.
In U.S. Pat. No. 3,239,620 a fluid damped velocity switch is disclosed. This arrangement includes an outer body with a cavity containing damping fluid, and an inner body disposed in the cavity and moveable relative thereto forming at least one variable volume chamber therein and damping means connecting to the chamber controlling flow therethrough. Disclosed in connection with this velocity switch is a helix and bellows arrangement used for temperature compensation; fluid is caused to flow through a variable groove length in the helix depending on the fluid velocity, which varies with temperature, under the control of the bellows.
A similar arrangement is used for temperature compensation in a multiple output intergating acceleromator disclosed in U.S. Pat. No. 3,771,368. In that patent, a plurality of proof masses which are adapted to move relative to a casing in response to acceleration of the casing are disposed in a damping fluid. The proof masses are of different mass or size so that they respond differently to forces on the device resulting from acceleration. Output signals are generated in response to predetermined movement of each of the proof masses. The principles on which this device operate are similar to those of U.S. Pat. No. 3,239,260. Once again, a helix and bellows arrangement is used for temperature compensation. A further device of this nature is disclosed in co-pending application Ser. No. 591,433 filed June 30, 1975 and assigned to the same assignee as the present invention. It too relies on a bellows and helix arrangement for temperature compensation.
Although the temperature compensation provided by such an arrangement works quite well during normal temperature changes problems can occur if sudden temperature changes occur during operation. Since such accelerometers are in missiles and the like it is most likely that such a temperature transient during operation would be caused by the radiation of a near-by nuclear event. Should the accelerometer of U.S. Pat. No. 3,771,368 be subjected to such rapid and intense heating it has been calculated that inaccuracies will occur.
The purpose of the bellows is in these devices is to reposition the helix as the fluid temperature, and thus the fluid volume changes. The helix is designed to present the correct resistance to flow such that the instrument scale factor remains constant with temperature. Under normal circumstances, the bellows and helix perform this function as desired. However, what will take place during a rapid heating such as will occur during a nuclear event must be considered. The fluid increases in temperature, expands and displaces the bellows upward. As the bellows moves upward the helix moves along with it. The helix displaces fluid which is blocking its motion. The fluid is constrained to move from an area above the helix through the bore containing the proof mass. Since a positive displacement of the fluid takes place, the ball or proof mass which is in the way of the fluid motion is displaced in the bore. With the arrangement shown in the previous patents, an upward movement of the bellows will result in a downward movement of one or more of the proof masses. To the instrument this is the same as seeing a certain number of G seconds when in fact it has not. Were the instrument to experience a sudden decrease in temperature while intergating, the bellows would expand and the ball would travel upward. If the instrument experienced a temperature change, but then return to its initial temperature before the intergration cycle is completed, the net bellows motion would be zero and no instrument error would result. However, such can not be depended upon.
Thus, it can be seen that there is a need for an improved temperature compensation mechanism to take care of sudden temperature changes.